Why is that? I think they did a lot of things right. Offer automatic conversions, backwards and forwards compatibility from a sufficiently recent 12.x version.

I think mostly Kotlin being simpler and Java gaining features ate the lunch. Also, software like Akka and Spark becoming less prevalent hurt because they were big reasons for devs to learn the language. Not to mention the community drama.

The only bad thing was that it took quite long for Scala3 to become available leading to a lot of stagnation.